I think that the (still incomplete) C API to Pika Scheme is, overall,
a better candidate for a portable FFI.

It can be found at:

      http://arch.quackerhead.com/~lord/releases/pika
      http://regexps.srparish.net/src/pika/

and there's a reference manual in there in .ps, .html, and .texi
formats.

-t